This invention relates in general to self-regulating heaters and more specifically to PTC ceramic heater devices particularly suitable for heating compressor oil.
In conventional refrigeration compressors, a refrigerant, such as one sold under the trademark "Freon" by E. I. dePont de Nemours, & Co., under certain temperature conditions tends to migrate from the condenser into the compressor lubricant. Such migration is undesirable since it causes deleterious effects including the reduction in lubricating properties of the lubricant. In order to avoid this problem it is conventional to employ a crankcase heater to maintain the compressor crankcase at a temperature above that of the rest of the refrigeration system which has the effect of boiling out any Freon already in the lubricant and of preventing migration of the refrigerant into the crankcase lubricant. Recently, improvements have been effected in these heaters making them self-regulating, thus improving their reliability while doing away with the costs involved in associated regulation controls. By way of example: U.S. Pat. Nos. 3,564,199; 3,720,807; 3,748,439; 3,824,328; 3,940,591; 3,996,447; 4,086,467; and 4,091,267 all disclose self-regulating heaters useful in many applications including the heating of compressor crankcases. These devices employ a heater made of ceramic material having a positive temperature coefficient (PTC) of resistivity. Such heaters have a relatively low resistance at normal ambient temperatures, but following energization by a source of electric power will self heat and increase in temperature and resistance. Once a theshold or anomaly temperature is reached the resistance increases rapidly by several orders of magnitude and will stabilize when the heat generated balances the heat dissipated. At this point the resistance level is many times the initial room temperature value.
While the heaters of the above mentioned patents are effective for many applications, it is an object of the present invention to provide a self-regulating heater and a method for making such a heater, which is more conducive to mass production assembly techniques than prior art devices. Another object is the provision of a self-regulating heating device which uses a minimal number of components and thus can be produced at a low cost while still producing such heaters which are reliable and efficient.
The self-regulating heater of this invention preferably comprises a cylindrical body of steatite or other electrically insulative ceramic in which a slot of parallelepiped configuration is formed extending in an axial direction from an open end toward a closed end of the cylindrical body. A single PTC resistor, formed of ceramic material such as a doped barium titanate, is configured slightly smaller than and complementary with the slot and is received therein. In two of the walls defining the slot, an axially extending groove is formed coextensive in length with the slot. Intermediate the ends of the groove a platform is formed in the body to serve as a stop surface to limit the extent that a spring biased terminal can be inserted. Insertion of the terminals in the grooves place them in electrical connection with spaced portions of the PTC resistor. The grooves are disposed on opposite sides of a plane in which the longitudinal axis of the cylinder lies and which is perpendicular to the walls in which the grooves are formed in order to optimize spacing between the leads. Automated assembly of the device includes the steps of sliding or inserting the resistor into the slot, inserting injection nozzles into the grooves until they are adjacent the closed end of the cylinder and injecting thermally conductive grease like material into the space between the resistor and the cylinder at the same time the nozzles are removed from the grooves, sliding a spring biased terminal into each groove until it bottoms against a respective platform and then dispensing a first sealing layer of RTV silicon in the open end of the cylinder around the two leads passing therethrough and a second layer of epoxy on top of the first layer to provide effective pull strength for the leads. If the device is assembled by hand the above procedure is modified by coating the grease like material on the resistor before sliding it into the slot. The remainder of the procedure remains the same.